1. Field of the Invention
The present invention relates to an electric power steering system for vehicles, and more particularly, to an electric power steering system for vehicles in which auxiliary torque for steering is produced by means of a steering servo device using an electric motor.
2. Description of Relevant Art
There have been proposed in recent years a variety of electric power steering systems for vehicles in which auxiliary torque for steering is produced by means of a steering servo device using an electric motor.
As an example, there is a steering system disclosed in UK Patent Application Publication GBA No. 2132950. The steering system includes a torque detection mechanism for detecting steering torque acting on an input shaft connected to a steering wheel, an output shaft connected to front wheels as steered wheels, a direct-current motor for giving auxiliary torque to the output shaft, and a control circuit for controlling the motor in dependence on a detection signal from the torque detection mechanism. FIG. 15 shows a motor drive circuit 200 equivalent to the control circuit. The drive circuit 200 consists of a bridge circuit 201 for applying an auxiliary torque producing electric motor 220 with an armature voltage Va and a drive unit 210 for feeding the bridge circuit 220 with drive signals. To the drive unit 210 is input a control signal 211 from an unshown signal processor. In the processor a steering torque representative signal is processed, following a given program, so that the control signal 211 has, for example, such a component as responsible for the direction of rotation of the motor 220 to be coincident with that of steering torque Ts acting on the input shaft. The bridge circuit 201 comprises npn type switching transistors 202, 203, 204, 205 constituting four arms of the bridge, respectively. Transistors 202, 203 and 204, 205 have diodes 206, 207 and 208, 209 connected in parallel thereto respectively, and inverse-parallel to the motor 220. An output lead of the circuit 201 extends from the node between neighboring two transistors 202, 203, and another from that between the remaining two transistors 204, 205. The output leads constitute a loop across the motor 220. Input leads, which extend from the positive and negative poles of a power supply of a constant voltage Vcc, terminate at the node between neighboring two transistors 205, 202 and that between the remaining two transistors 203, 204, respectively. The drive unit 210 has four output terminals connected to the bases of transistors 202 to 205, respectively. The transistors 202 to 205 are driven by the drive unit 210 in accordance with contents of control signal 211. For example, if the signal 211 is responsible for causing the motor 220 to rotate in such a direction A as corresponding to clockwise acting steering torque Ts, then the transistor 202 is driven on in a continuous manner and the transistor 204 in a PWM (pulse width modulation) control manner. Transistor 204 is then applied at the base with a PWM signal which is a frequency-constant pulse signal of a rectangular wave with a level of supply voltage Vcc, as modulated with respect to the pulse width, i.e., controlled of its duty D (proportion of pulse duration) in dependence on a component of the control signal 211. To the contrary, if the signal 211 is for rotating the motor 220 in the opposite direction B which corresponds to counterclockwise acting steering torque Ts, the transistor 205 is driven on in a continuous manner and the transistor 203 in a PWM control manner. The duty D of the PWM signal applied to transistor 204 or 203 has a value determined in proportion to the magnitude of steering torque Ts. The motor 220 is applied with an armature voltage Va of which the magnitude is proportional to such a value of duty D. As a result, the armature voltage Va the bridge circuit 201 applies across the motor 220 has the magnitude properly controlled as well as the polarity thereof, with the transistors 203, 204 either PWM driven.
FIGS. 16A and 16B show circuit diagrams substantially equivalent to the bridge circuit 201 working with the transistors 202, 204 driven in combination so as to rotate the motor 220 in the direction A in correspondence to clockwise acting steering torque Ts. Transistor 202 is continuously driven on, and transistor 204 PWM driven. In strict accordance with the cited UK Publication, of two switching transistors working in combination, one to be PWM driven is put between the positive pole of a battery and an electric motor and the other to be continuously driven on is between the motor and the negative pole of the battery. Such differences may well be neglected.
The circuit of FIG. 16A corresponds to a state in which the transistor 204 is turned on with the PWM signal sent thereto at the level Vcc. With power supply 230, transistor 202, motor 220, and transistor 204, there is formed a close circuit, which conducts an electric current across the motor 220, i.e., an armature current Ia. The armature current Ia has a magnitude corresponding to a load imposed on the motor 220 from the road surface, via the output shaft. The output shaft connected to the front wheels, as steered road wheels, is mechanically interconnected through a rack and pinion mechanism and the like with the steering wheel. Thus, a later-described restoring force Fr, acting as a load in the form of a moment from the road surface side, the motor 220 bears such a share as resulted by subtracting the steering torque Ts from the load Fr. FIG. 16B represents a state when the transistor 204 is turned off. A close circuit is formed with transistor 202, motor 220 and diode 209 so that there is a transient armature current of the de-energized motor 220. Like the armature curent Ia in the close circuit of FIG. 16A, the transient armature current is interlinked with the load on the motor 220, through an internal inductance of the motor 220 which induces a counter emf (electromotive force) Vi in dependence on the rpm (number of revolutions per minute) of a rotor having a certain mass of inertia.
As well as in steering systems without power assist, the steering wheel in the above power steering system is put in either of two different states when it rotates, i.e., a positive steering state or a returning state. In the positive steering state, the steering wheel has steering torque Ts applied thereto in such a direction as coincident with the turning direction of the steered wheels, and hence with the direction of rotation of the motor 220 as well. In the returning state of the steering wheel, however, the acting direction of steering torque Ts applied thereto is not coincident with, but opposite to, the turning direction of the steered wheels so that the rotating direction of motor 220 is opposite to the acting direction of the steering torque Ts.
When a traveling vehicle turns with front wheels as steered wheels thereof turned in either direction by a steering system, the front wheels have a system of forces produced by their wheel alignment and self-aligning torque due to deformations of their tires. A collective resultant of such forces acts as a couple on the front wheels with a tendency to return the wheels to their neutral positions, and is called a "restoring force". The restoring force is transmitted in the form of a moment to the steering system, and applied thereon as a load from the road surface.
In a vehicle equipped with the electric power steering system described, when it turns while traveling, if the front wheels are acted on by a restoring force Fr developed, for a certain reason, larger in magnitude than the sum of steering torque Ts applied to the steering wheel and auxiliary torque output from the motor 220, the steering wheel is caused to rotate, with a moment transmitted from the front wheels, in the opposite direction to the acting direction of the steering torque Ts. Also the motor 220 is forced to rotate in a reverse direction, which corresponds to the direction B in the case the vehicle turns to the right, for example. Such a situation sometimes takes place when the magnitude of steering torque Ts, applied by the driver to the steering wheel, is small, while the vehicle is turning, with an intention to leave the front wheels returning toward the neutral positions with a restoring force Fr acting thereon. This is a typical example of the steering wheel returing state.
Steering torque Ts is now supposed to be clockwise acting, so that the transistor 202 is continuously driven on, and the transistor 204 PWM driven with a PWM signal of which the duty D has a value determined in accordance with the steering torque Ts. While the transistor 204 is on, the armature current Ia is conducted as shown by arrows in FIG. 16A. During the positive steering state of the steering wheel, the motor 220 rotates in the direction A, producing auxiliary torque that permits a more comfortable steering operation than would be achieved in a steering system without assist power. In the steering wheel returning state, however, the motor 220 is rotated in the direction B and functions as a generator. The returning speed of the steering wheel toward its neutral position may thus be appreciably slower than expectable in the non-assisted steering system.
In this respect, the transistors 202, 204 themselves are controlled to be driven in combination to rotate the motor 220 in the direction A, whenever steering torque Ts acts clockwise, how small its magnitude would be. In actual, the magnitude of steering torque Ts is very small in the steering wheel returning state and hence the value of the duty D of the PWM signal, as well. This results in a very large fraction of pulse period to be shared for the equivalent circuit of FIG. 16B to substitute for the circuit of FIG. 15, whereas the duration of substitution of that of FIG. 16A becomes even shorter. The close circuit 220-209-202 of FIG. 16B is thus maintained over such a large fraction of pulse period.
In the steering wheel returning state, the motor 220 rotating in the direction B has a counter emf Vi induced thereacross by generator action with such a polarity that it increases the total emf of the close circuit of FIG. 16A. When the transistor 204 is turned off, such a counter emf Vi produces in the close circuit 220-209-202 of FIG. 16B a transient armature current Ia' of which the flow direction with respect to the motor 220 is the same as that of the armature current Ia, as shown by arrows in FIG. 16B. Permitted conduction of such an armature current Ia' through the close circuit 220-209-202 implies that the front wheels have to perform an appreciable work. The front wheels thus tend to be unsmooth in returning to the neutral positions. If the vehicle speed is constant while turning, the restoring force Fr is at the maximum when the front wheels starts returning to the neutral positions, and becomes smaller as they approach the same. The above tendency is thus significant in an initial phase of the steering wheel returing state. In the steering wheel returning state, the diodes 207, 209 in the drive circuit 200 of FIG. 15 are forward connected with respect to the armature current Ia' so that there is formed another close circuit with motor 220, diode 209, power supply 230, diode 207, and unshown resistors and the like. Such a close circuit however is neglectable in the present discussion.
Discussion is continued from a theoretical viewpoint, with reference to FIG. 17. The axis of abscissa represents the duty D of the PWM signal to the transistor 204, and that of ordinate the armature current of the motor 220 in terms of effective value (collectively designated by Ia). On the D-Ia plane, there is plotted characteristic curves of the motor 220, with the motor rotation speed Nm as the parameter. In the graph, DZ is a dead zone for the drive circuit 200 to drive the motor 220 to rotate in the direction A. Nmi (i=1 to 4) is a particular rotation speed in terms of rpm of the motor 220, as algebraically represented such that the motor 220 is rotated in the direction A when Nmi is positive or has no signs, and in the direction B when it has a negative sign. Where Nmi is signed negative, therefore, the steering wheel is in the returning state. Steering torque Ts is still supposed to be clockwise acting. An armature current Ia1 is needed to be conducted through the motor 220, against a restoring force Fr1 acting on the front wheels. Under such a condition, if the rotation speed Nm is desired to be controlled at zero (Nmi=0), the duty D should be set to such a value D1 that a resulted effective voltage as an armature voltage Va to be applied to the motor 220 permits the armature current Ia1 to flow without counter emf Vi induced across the motor 220. To have the motor 220 rotate with a positive rotation speed Nm3, the duty D should be selected to be a value D2 such that D2=D1+a, where a is a necessary increment of duty D corresponding to that of armature voltage Va for cancelling the effect of an induced counter emf Vi when the rotation speed Nm is raised (from 0) to Nm3. Such a counter emf Vi otherwise would decrease the armature current Ia (from Ia1). To have the motor 220 rotate with a negative rotation speed -Nm3, there should be selected a duty value D0 such that D0=D1-b, where b is a necessary decrement of duty D for keeping the armature current Ia from being increased by an induced counter emf Vi at the rotation speed -Nm3. The duty value D0 however lies within the dead zone DZ, and cannot be set by the drive circuit 200. If the value Fr1 of restoring force Fr is one in the initial phase of the steering wheel returning state, the restoring force Fr becomes smaller than Fr1 as the front wheels approach the neutral positions, and hence the armature current Ia gradually decreases therewith from the value Ia1. To control the motor 220 at the negative rotation speed -Nm3 when a smaller armature current than Ia1 is conducted therethrough, the duty D must be a value not exceeding zero. The drive circuit 200 however is unable to set below zero the duty D of the PWM signal which is sent to the transistor 204, whenever steering torque Ts is clockwise acting.
The present invention has been achieved to effectively solve such problems in the prior art described.